(1) Technical Field
This invention relates to the formation of metal parts by the use of foam patterns which are consumed or evaporated during the casting of the metal followed by controlled heat treatment in a single reactor.
(2) Description of the Prior Art
The casting or molding of metals is an art that has been in existence for a very long time and yet, surprisingly, has not experienced very many changes with respect to the basic techniques or materials used in the process.
Most of the prior art casting processes such as green sand molding or permanent molding require subsequent heat treatment in a separate operation to achieve the desired microstructures.
A typical prior art process for making a casting is by the "lost foam" or "evaporative pattern casting" method. This method initially requires the formation of a foam pattern out of a consumable polymeric material.
This foam pattern is then dipped or coated with a ceramic slurry and placed in a bed of dry, loose, cool sand. The sand bed is thoroughly vibrated to compact the particles around the coated pattern. After the compaction is completed, molten metal is poured directly onto the polymeric pattern. The pattern gradually evaporates as it comes in contact with the molten metal. The products of evaporation, mostly gases, are vented into the dry sand bed, leaving behind a cavity for the molten metal to fill. As the evaporation, followed by filling by molten metal, is completed, an exact replica of the polymeric pattern is reproduced in metal.
This typical prior art process of "lost foam" casting has problems in that the formation of the ceramic shell is costly and time consuming. The porosity of the ceramic shell also has to be carefully controlled in order to allow the gases evolved during the evaporation of the polymeric pattern to exit through the shell. Another problem with this conventional process is that the polymeric pattern often decomposes into lustrous carbon and gases which become defect core impurities in the metal part. In order to address this problem, the density of the pattern has to be carefully regulated, which also involves the expenditure of additional time and monitary expense.
There also exists the possibility that the ceramic shell may warp or crack because of the introduction of the molten metal therein. This would necessitate the formation of a new ceramic shell which would entail additional expenditures of time and expense.
In another "lost foam" or "evaporative casting" process, an oven is used to burn out the polymeric foam pattern from the ceramic shell. This process also requires the polymeric foam pattern to be coated with a large number of coats of the ceramic material since the ceramic shell ultimately must support the molten metal, and the problems also arise of damaging the ceramic shell during its removal from the oven and the ceramic shell warping during the burning out of the foam pattern.
Another prior art process for making a casting is the "lost wax" or "investment casting process". The "lost wax" process entails the coating of a wax pattern with a ceramic slurry. The coated wax pattern is inserted into a steam autoclave to remove the pattern. The removal of the wax pattern weakens the ceramic shell so the ceramic shell must be heated at temperatures up to 2000.degree. F., in order to strengthen it. Molten metal is then introduced into the ceramic shell in order to form a casting.
Due to the weakness of the ceramic shell, the subsequent handling of the shell in order to introduce the molten metal therein, and the need for the ceramic shell to have sufficient strength to contain the molten metal without external support, from 10-14 coats of the ceramic slurry are applied to the wax pattern. The application of the large number of ceramic coatings consume a great deal of time and money.
U.S. Pat. No. 3,572,417 discloses a method for casting or molding metals in a mold. The mold comprises a refractory inorganic oxide foam which has been formed by heating a filled organic foam at a temperature and time sufficient to substantially decompose an organic binder to a carbonaceous state, or, alternatively, to substantially consume the organic binder to form a refractory inorganic foam. This patent discloses the use of an oven to decompose the organic binder and to fuse or sinter the remaining inorganic components. However, the formation of the "green" mold is an extremely complicated process and the time and expense involved in heating the "green" mold to a temperature sufficient to decompose the organic binder and fuse the remaining inorganic components is unnecessarily high.
U.S. Pat. No. 4,115,504 discloses a method for casting vitreous materials using the lost wax process. In this patent, a pattern of the article to be cast is made using a substance which is vaporized during the casting. Wax, polystyrene and polyethylene are disclosed as being suitable materials for the pattern. The pattern can be coated with a thin layer of a mixture of graphite and a refractory powder and then embedded in a heat resistant silica compound to form a casting mold. The silica compound typically is moistened or contains a cohesive material, such as a resin, in order to insure that the portion of the sand mold adjacent to the surface of the cast articles dries thoroughly. A vitreous material having a viscosity of between 20 and 100 poises is introduced into the mold and decomposes the pattern. The article formed in the mold can be ceramified by fluidizing the sand by means of a hot stream of gas. It is to be noted that this reference deals with the formation of a vitreous article, not a metal article, and requires that the viscosity of the casting material be maintained in a certain range in order for the casting material not to pierce the mold.
U.S. Pat. No. 4,640,728 discloses a method of joining foam patterns which are used in evaporative casting processes. This patent discloses a method of assembling complex, consumable foam patterns for use in evaporative pattern metal casting but gives no particulars as to the evaporative pattern process per se.
U.S. Pat. No. 4,995,443 discloses a method for producing a cast metal object in which a heated particulate medium at a temperature of between 1500.degree.-2000.degree. F. is used to support a ceramic shell containing a consumable polymeric pattern. The heat from the heated particulate medium causes the polymeric pattern to decompose and vaporize and form a cavity in the ceramic shell into which molten metal may be introduced. The molten metal is then allowed to solidify and form the cast metal object. Due to the high temperature of the particulate medium in this method, problems arise with respect to the handling and disposal of the vapors from the decomposed pattern.
In the production of many cast articles of steels, cast irons and some nonferrous alloys, the casting is subsequently heat treated in a subsequent operation to achieve a desired microstructure. This subsequent heat treatment is very expensive in terms of energy consumption, time and handling costs.
It is an object of the present invention to provide a method for forming a cast metal article by an evaporative casting process which does not contain the drawbacks of the processes used in the prior art.
It is a still further object of the present invention to provide an evaporative casting process followed by controlled heat treatment in a single reactor in an economical manner.
It is a still further object of the present invention to provide a process in which wasteful energy consumption in transporting cast parts to a heat treatment shop and repeated handling is eliminated. And as a result, economical production is achieved.